The default national language setting (NLS) used in mbstring. Note that this option
automagically defines mbstring.internal_encoding and
mbstring.internal_encoding should be placed
after mbstring.language in php.ini

According to the » HTML 4.01 specification,
Web browsers are allowed to encode a form being submitted with a character
encoding different from the one used for the page.
See mb_http_input() to detect character encoding
used by browsers.

Although popular browsers are capable of giving a reasonably accurate guess
to the character encoding of a given HTML document, it would be better to
set the charset parameter in the
Content-Type HTTP header to the appropriate value by
header() or
default_charset ini setting.